When skeletal muscle is forcibly lengthened while activated (eccentric contraction), injury occurs to the muscle that is characterized by a rapid and prolonged loss in force-generating ability followed by delayed onset muscular soreness. Injury to skeletal muscle from eccentric contractions is an extremely common clinical condition that occurs as a result of vigorous exercise or other forms of normal and accidental muscle overuse. Although recent studies have provided some important insights into the cellular and biochemical adaptations that follow eccentric contraction-induced injury, the precise mechanical conditions, at the level of the sarcomere, that result in mechanical injury remain poorly understood. For example, studies from various animal models have reported conflicting results as to whether mechanical stress or strain is the primary determinant of muscle injury. Also, the precise influence of initial sarcomenre length and lengthening velocity on injury remains poorly characterized. Further, only indirect evidence exists as to whether fibers of a particular size and type are selectively predisposed to mechanical injury. Although it has been theorized that non-uniformity in sarcomere length during eccentric contractions leads to mechanical instability, sarcomere popping and subsequent injury, direct tests of this theory have been difficult to achieve. Our understanding on the mechanical basis of muscle injury has been limited because most studies have been performed on whole muscles. The problem with whole muscle preparations is that sarcomeric strain and mechanical stress in individual fibers cannot be measured directly or predicted accurately. In contrast, single isolated fibers allow for accurate measurement of sarcomere strain along the entire length of the cell, and thus permit precise correlations to be made between mechanical events and contractile performance. However, because single intact fibers are extremely difficult to isolate from mammalian muscle, single fiber studies in mammals are restricted to skinned fiber preparations, where the cell membrane is disrupted, drastically altering the fiber's mechanical properties and cell signaling pathways. In contrast to mammalian muscle, single intact fibers can be readily isolated from frog muscle that retain complete mechanical stability, making it possible to study mechanical injury in an intact cellular environment at the single fiber level. Thus, the purpose of this proposal is to study mechanical-based muscle injury during eccentric contractions using intact frog single fibers. The mechanics experiments will be performed while monitoring segment length and sarcomere length transients along the full length of the fiber, providing a very precise and high resolution correlation between mechanical events (fiber stress and sarcomeric strain) and muscle injury. The extent of sarcomere popping will also be evaluated. The mechanical events that result in injury will be related to fiber size and fiber type at the single fiber and individual segment level. The aged population is particularly susceptible to the debilitation effects of eccentric contraction-induced injury because of general muscle weakness and poor regenerative properties. An understanding of the precise mechanical conditions that result in muscle injury could lead to improvements in the development of preventative therapies and rehabilitation.